The public IP address will change when you reboot or stop and start the instance in Lightsail. This does not match our demands. So we need a static IP address for our vps.
Just follow these steps:
1. Home->Networking->Create static IP
2. Home->Instances->Manage->Networking->Attach static IP
3. Select a static IP
If want to use HTTPS for your site, you need to open the 443 port manually.